Examples of active contactless information storage devices having batteries therein include contactless IC (integrated circuit) cards, contactless IC tags, and RFID (Radio Frequency IDentification) tags. For example, RFID tags are attached to products and are carried by users. The RFID tags transmit information regarding the products or the users by using RF (radio frequency) signals with a predetermined frequency. The RF signals transmitted from the RFID tags are read by a reader device having a reading function and are processed by, for example, a host apparatus, such as a computer, so that the resulting information can be used, for example, for monitoring and managing distribution of the products or actions of the users. A writer device having a writing function may be used to transmit and write predetermined information to the RFID tags.
The active RFID tags having batteries therein can communicate over a relatively long distance, compared to passive RFID tags that contactlessly receive power from, for example, a reader/writer device having reading and writing functions. The active RFID tags are broadly classified into periodic-spontaneous-communication-type RFID tags and standby-communication-type RFID tags. The spontaneous-communication-type RFID tag is configured to perform communication at regular intervals based on a clock or the like built into the RFID tag and to sleep in other periods of time, thereby reducing the power consumption. On the other hand, the standby-communication-type RFID tag is configured to wait for, for example, a call (a radio wave) from the reader/writer device to perform communication, without initiating the communication.
In order to sense the call from the reader/writer device, the standby-communication-type RFID tag is adapted to constantly perform carrier sense at regular intervals. The carrier sense involves a period for sensing a signal from the reader/writer device and a period for detecting a signal from other active RFID tags that are present in a range in which they can communicate with the reader/writer device. Provision of the latter carrier-sense period makes it possible to prevent, when one active RFID tag starts transmission to the reader/writer device while another active RFID tag is performing transmission to the reader/writer device, radio-wave interference between the RFID tags from occurring. Since the active RFID tag performs carrier sense at regular intervals in periods other than when communicating with the reader/writer device, as described above, the power consumption is relatively large and the amount of load on the battery in the active RFID tag is relatively large (e.g., Japanese Unexamined Patent Application Publication No. 2006-338489).